1. Field of the Invention
The present invention relates to a photoemission (photoelectron-emitting) device having excellent quantum efficiency in photoelectric conversion (hereinafter referred to as quantum efficiency), an electron tube with a photoelectron multiplying function, such as a photomultiplier tube or an image intensifier, employing the photoemission device to achieve increased sensitivity, and a photodetecting apparatus with high sensitivity employing such an electron tube.
2. Related Background Art
Photoemission devices convert incident photons into photoelectrons and emit the photoelectrons to the outside, and, for example, are applied to light-receiving surfaces of photomultiplier tubes or image intensifiers.
Materials such as alkali antimonides are generally used in conventional photoemission devices. For example, monoalkali photoemitters such as Sb.multidot.Cs, bialkali photoemitters such as Sb.multidot.K/Cs, and multialkali photoemitters such as (Na.multidot.K.multidot.Sb)Cs are widely put to practical use. The photoemitters of such types, however, had a lower photoemission ratio (quantum efficiency for long-wavelength incident photons than that for short-wavelength incident photons, which raised a problem that high-sensitive performance could not be achieved over a wide band and a problem that even for short-wavelength incident photons the quantum efficiency was not high enough.
In order to improve the quantum efficiency for long-wavelength incident photons, negative electron affinity photoemitters using a GaAs semiconductor were developed. In the negative electron affinity photoemitters, the energy of the vacuum level is lower than the conduction band. Then, once photoelectrons at the bottom of the conduction band can move up to the emission surface, they can escape into the vacuum. This can improve the quantum efficiency for long-wavelength incident photons. Use of a single-crystal semiconductor of GaAs can extend the diffusion length of photoelectrons as compared with the photoemitters using polycrystal materials of alkali antimonides. Even if the single-crystal semiconductor is thick enough to absorb all incident photons, the diffusion length can be too long for photoelectrons to reach the emission surface.
Actual quantum efficiencies of the negative electron affinity photoemitters, however, are still about 20% for the wide band ranging from short wavelengths to long wavelengths, though an improvement is recognized for long-wavelength incident photons.
As discussed, the quantum efficiencies of the photoemitters under practical use are about 30% for short-wavelength (for example, ultraviolet) light, but normally about 10%, which is extremely low as compared with known solid state photodetectors such as photodiodes utilizing the photoconduction or photoelectromotive force. This is a significant drawback in light detection technology utilizing photoemission, because approximately 90% of all photons incident into the photoemission device are not detected.
Further, it is generally known that with negative electron affinity photoemitters the quantum efficiency can be increased by such an arrangement that the anode is located in close proximity to the emission surface of photoelectrons and a high voltage is applied between them to generate a high electric field near the emission surface. It is structurally difficult, however to form a uniformly narrow gap between the anode and the cathode (pole on the emission surface side) in order to obtain such a high electric field. If an applied voltage is increased instead of narrowing the gap, a high-voltage power supply of about 10 kV is necessary, raising a problem of electric discharge caused between the emission surface and the anode.
U.S. Pat. No. 3,958,143 discloses another example of a conventional photoemitter. In this photoemitter, a Schottky electrode is formed on one surface (photon-entering surface) of a photon absorbing layer of a semiconductor or a semiconductor hetero structure, and an ohmic contact is formed on the other surface (opposite to the photon-entering surface with respect to the photon absorbing layer). When photons enter the photon absorbing layer and a bias voltage is applied between the Schottky electrode and the ohmic contact at predetermined polarities, photoelectrons excited in the photon absorbing layer move to the Schottky electrode and are transferred to a higher energy band to be emitted into the vacuum.
The photoemitter of such structure was achieved with a Schottky electrode formed from a very thin (below 100 angstroms) Ag film. Accordingly, even existing semiconductor fabrication technology can rarely assure reproducibility and uniformity of the film thickness of the Schottky electrode, presenting great difficulties in putting this technology to practical use.
Japanese Laid-open Patent Application No. 4-269419 discloses another photoemitter which attempts to solve the problem in U.S. Pat. No. 3,958,143. In the photoemitter, a Schottky electrode is formed in a suitable pattern on one surface (photon-entering surface) of a photon absorbing layer of a semiconductor or a semiconductor hetero structure, and an ohmic contact is formed on the other surface (opposite to the photon-entering surface with respect to the photon absorbing layer). When photons enter the photon absorbing layer with a bias voltage being applied between the Schottky electrode and the ohmic contact at predetermined polarities, photoelectrons excited in the photon absorbing layer move to the Schottky electrode and are transferred to a higher energy band to be emitted into the vacuum. Thus, Japanese Laid-open Application No. 4-269419 employed a patterned Schottky electrode instead of a uniform Schottky electrode formed over the entire surface of the photon absorbing layer, thus enabling the uniformity and reproducibility to be enhanced through in the use of the lithography technology. In other words, Japanese application No. 4-269419 succeeded in improving the uniformity and reproducibility of the Schottky electrode. The photoemitter, however, had a problem that the sensitivity (quantum efficiency) for long-wavelength incident photons was lower than that for short-wavelength incident photons.
An object of the present invention is to provide a photoemission device showing high-sensitive performance over a wide wavelength range and further to provide an electron tube and a photodetecting apparatus employing such a photoemission device.